A. Field of the Invention
This invention relates to the measurement of differential pressure. More particularly, this invention relates to such measurements made by use of a resonant sensor developing an output signal the frequency of which is responsive to an applied differential pressure.
B. Description of the Prior Art
Differential pressures have been measured for many years, in large part for the purpose of determining fluid flow rates in industrial processes by measuring the differential pressure produced across an orifice plate in a flow pipe. Such measurements typically are made by apparatus commonly referred to as a differential-pressure cell, of which there are a number of different types.
One type of differential-pressure cell, which has gone into widespread commercial use, employs a vibrating wire tensioned by a force corresponding to the differential pressure so that the resonant frequency of vibration of the wire reflects the magnitude of the differential pressure (see U.S. Pat. No. 4,165,651 to E. O. Olsen et al). The wire vibration is detected by electronic circuitry which develops a corresponding output signal suitable for transmission to a remote location such as a contro1 station or the like. One important advantage of such resonant sensors is that the output signal can be developed directly in pulse format, and thus can readily be adapted to digital circuitry which now is coming to be used extensively in instrumentation systems.
Differential pressure cells for commercial application must be capable of making measurements at quite high static (common mode) pressures, e.g. up to 6000 psi. Commonly, to measure such high pressures, the apparatus comprises a capsule forming a liquid-filled chamber having its ends sealed by pressure-responsive diaphragms which couple the process pressures to the interior of the chamber. The sensing element (e.g. the vibrating wire in the above U.S. Pat. No. 4,165,651) is in the fill-liquid capsule, and the differential pressure coupled through the diaphragms develops a corresponding stress on the sensing element. The fill-liquid is essentially incompressible, and withstands the high static pressures involved without allowing damage to the flexible diaphragms or the like used to transmit the differential pressure through to the sealed interior chamber.
The vibrating-wire resonant element described in the above-mentioned U.S. Pat. No. 4,165,651 is capable of operating in a fill-liquid without serious adverse effects resulting from the presence of the liquid around the transversely vibrating wire. However, its performance would be improved if not surrounded by a liquid. Moreover, other types of resonant elements, such as certain resonant beams or tuning forks having important advantages as sensing elements, will not perform satisfactorily when surrounded by liquid. Thus, there are benefits to be obtained by operating vibratory sensing elements of a differential-pressure cell in a vacuum, or at least in a gaseous medium at a low pressure such as atmospheric. Among such benefits is that of making it possible to employ as sensing elements the recently available miniaturized vibratory beams formed by photolithographic processes, e.g. the resonant element known as a double-tuning fork (DTF). For further information on such resonant elements, reference may be made to U.S. Pat. Nos. 4,215,570 and 4,372,173.
If the resonant element is to be in vacuum, or in a gaseous medium at low pressure, then it must be isolated from the high process pressures by some form of pressure-resistant barrier. Thus, the flexible pressure-responsive transmitting devices such as diaphragms, previously used to couple forces to a sensing element in the interior chamber of a liquid-filled capsule, can no longer be used for that purpose. That is, it becomes necessary to employ other than conventional flexible means for transmitting the applied differential pressure through a barrier to the sensing element.
The differential-pressure instrument also preferably should be arranged to accommodate optical excitation and read-out of the resonant element. Such arrangements (see for example, U.S. Pat. No. 4,521,684 to Gilby et al) are very well suited for use with the tiny beam-like resonantors now becoming available, and which may make possible the development of greatly superior, minaturized measuring apparatus for industrial processes.